Microsoft's decision to limit Internet explorer 9 support to Windows Vista and Windows 7 prompted criticism from Mozilla, whose rival Firefox browser continues to support the still widely-used Windows XP.

A Mozilla executive last week said he was surprised that rival Microsoft has abandoned support for the 10-year-old Windows XP operating system in its new Internet Explorer 9 (IE9) browser.

IE9, which Microsoft launched a week ago today, runs on Windows Vista and Windows 7, its 2007 and 2010 operating systems, but not on 2001's XP. Microsoft has said the decision to not support Windows XP came out of its move to accelerate some browser chores, including composing the page, by tapping the graphics processor, or GPU.

"We knew we didn't want to optimize for the lowest common denominator, you need a modern operating system," said Ryan Gavin, senior director of IE, in an interview last week . "[Supporting XP would have been] optimizing for the lowest common denominator. It's ten years old. That's not what developers need to move the Web forward."

Microsoft and Mozilla have argued about the path to hardware acceleration each has taken, with the former claiming IE9 is the only browser that "fully hardware accelerates the entire Web platform," a claim some at Mozilla have disputed.

Johnathan Nightingale, the director of Firefox, refused to be drawn into the debate over which browser sported the best hardware acceleration on Windows Vista and Windows 7. Instead, he pointed out that Microsoft dropped XP.

"For me, the most interesting thing is not the quibbling about what browser [boasts] full hardware acceleration," Nightingale said in an interview last Thursday. "What surprises me the most is that acceleration is not available for Windows XP."

Both IE9 and Firefox 4 support hardware acceleration on Vista and Windows 7 by calling on those operating systems' Direct2D and Direct3D APIs for content rendering and compositing. Windows XP, however, does not support Direct2D, leaving Mozilla to partially accelerate Firefox 4 using Direct3D on the aged OS.

"That took us a lot of work. We had to do almost twice the work to accelerate [Firefox 4] on Vista and Windows 7, and Windows XP," said Nightingale. "But by our count, Windows XP still accounts for 40% to 50% of the Web. Our obligation is to the users, and Windows XP is not a part we can cut out."

Statistics from Web analytics firm Net Applications show that Windows XP accounted for 55% of all operating systems used to connect to the Internet last month, making it the most popular OS by far. When only those systems running Windows were tallied, XP's share jumped to 61%.

By comparison, Vista and Windows 7 together accounted for 38% of all versions of Windows used to reach the Web last month.

Other browser makers have taken Mozilla's tack, and will continue to support Windows XP. The newest versions of Google's Chrome, Apple's Safari for Windows and Opera Software's Opera all run on XP.

Mozilla will launch the final version of Firefox 4 , the browser that was once slated to debut in November 2010, on March 22 at approximately 7 a.m. PDT.

Last Friday, Mozilla decided to build a second release candidate, or RC, edition. "Two issues popped up which have caused us to take two very small, very isolated fixes in order to better protect Firefox 4 users," said Mike Beltzner, the outgoing director of Firefox, in a message to a company mailing list on Thursday.

According to Mozilla, the last-minute RC2 blacklisted several invalid SSL certificates and added a Vietnamese-language version to a list of more than 80 localized editions.

RC2 will not impact tomorrow's ship date, Beltzner said. "We still expect to release the final version of Firefox 4 for Windows, OS X and Linux on March 22 as planned," he said.

Firefox 4 will be available for download Tuesday from Mozilla's site. People running Firefox 3.5 or Firefox 3.6 can grab the upgrade by selecting "Check for updates" from the Help menu.